System and method for high-speed laser detection of ultrasound

ABSTRACT

A system and method for laser light amplification provides amplification of a laser light beam emitted from a laser light source as low-amplification seed laser light signal. The low-amplification seed laser light signal is transmitted to an amplification component. The amplification component amplifies the low-amplification seed laser light signal by stimulating emissions of the population inversion provided by a pumping diode to generate an amplified laser light signal. The system and method further directs the amplified laser light signal to an output destination. The destination may be an object undergoing laser ultrasound testing. The amplified laser light may reflect with a modulation characteristic of a sound energy wave about the object. The reflected laser light may be collected by an interferometer and used in the detection and characterization of the sound energy wave. The result of the present invention is a system and method of operation providing higher pulse rates, improved pointing stability, and optionally variable pulse rates for a variety of uses, including for nondestructive laser ultrasonic testing of materials.

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/344,298, filed Jun. 14, 1999 entitled: “Systemand Method for High-Speed Laser Detection of Ultrasound”, and isincorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE INVENTION

[0002] The present invention relates generally to a system and methodfor laser light amplification and, more particularly, to a system andmethod for providing amplification of a laser beam emitted from a solidstate laser that provides higher pulse rates, improved pointingstability, and optionally variable pulse rates for a variety of uses,including for nondestructive laser ultrasonic testing of materials.

BACKGROUND OF THE INVENTION

[0003] Amplification of laser light is required for a variety ofapplications. Long haul telecommunication applications, such as thoseemploying single mode optical fiber, often require opticalrepeater/amplifiers to boost sagging signal levels. Material processingapplications may require very high power laser light to performfunctions such as cutting of various materials and preparation ofmaterial surfaces. Applications requiring intense energy pulses of laserlight employ some configuration for providing either time-varyingoptical amplification or intensity modulation of laser light.

[0004] One method for amplifying a laser beam is to employ a lasermedium whose optical gain may be controlled by optical pumping. Opticalpumping of a solid state laser medium is a common and conventionalmethod used to create a population inversion of energy states for laserapplications requiring high-gain. The laser medium providing high-gain,when optically pumped, may comprise a material such as neodymiumyttrium-aluminum garnet (Nd³⁺:YAG), neodymium glass (Nd³⁺:glass), Erbiumdoped optical fiber (Er³⁺:silica), or Ruby rods (Cr³⁺:Al₂O₃). Thesematerials are merely exemplary candidates for high-gain laser media, andthose skilled in the art will appreciate that any suitable materialcapable of maintaining an inverted population of energy states whenoptically-pumped may serve as an optical amplifier. Those laser mediautilizing Nd³⁺:YAG are common, given the substantial optical gain neardesired wavelengths near the 1.064:m range. Additionally, Nd³⁺:YAG lasermedia provide linearity of pumping rate with respect to invertedpopulation given its four-level transition system.

[0005] To saturate an entire laser medium with an inverted populationthrough optical pumping, a conventional method is to distribute a largearray of laser diodes across the surface of the laser medium to form apumping array. The light emitted from the individual laser diodes of thepumping array excites the laser medium and provide a very high opticalgain for the energy transition level of the optically-pumped, invertedpopulation within the high-gain laser medium, e.g., near the 1.064:mrange for Nd³⁺:YAG, near the 1.06:m range for Nd³⁺:glass, near the0.6943:m range for Cr³⁺:Al₂O₃, near the 1.55:m range for Er³⁺:silica,etc.

[0006] An integrated approach to performing laser light amplificationand generating optical pulses utilizes gain switching of a laser medium.In this method of providing a high energy pulsed laser beam, the opticalpumping of a high-gain laser medium itself is pulsed to generate a timevarying gain of the high-gain laser medium through which a laser beam ispropagating. This results in a pulsed output laser beam after anoriginal laser light source has traveled through the high-gain lasermedium that is being optically pumped in a time varying manner.

[0007] Each optical pumping cycle takes the high-gain laser mediumthrough a transition which consists essentially of generating asufficient energy state population through optical pumping to reachthreshold for amplification. Before the optical pumping begins, thepopulation of energy states is initially below threshold and opticalamplification does not occur. After the high-gain laser medium hasoperated in an amplification mode for some time, then optical pumping isswitched off, and the energy state population is subsequently depleted.By turning off the optical pumping, the population falls below thresholdand the optical amplification is interrupted until the optical pumpingagain resumes and the population of energy states again reachesthreshold. Such a method provides for a pulsing of the conditions inwhich laser light amplification may occur. Such a method is preferableto a method which merely blocks a highly amplified laser beam in thatdesign considerations need not include the potentially loss energy dueto the dumping of electromagnetic energy into a shutter assembly. Manyother advantages are inherent to the fact that the solution iselectronic, not incorporating any mechanical components for a mechanicalshutter system.

[0008] Another method for providing an electronic solution is togenerate a high energy pulsed laser beam to maintain continuous opticalpumping of the high-gain laser medium and to modulate the high-gainlaser medium's loss coefficient. One method to perform such lossswitching is to electronically modulate an optical absorber that isplaced within the optical resonator cavity next to the high-gain lasermedium. Such a configuration will permit the user to control the loss ofthe laser light traveling through the high-gain laser medium as opposedto controlling the rate at which optical pumping occurs. Those skilledin the art will recognize a variety of methods for performing lossswitching of laser light contained within a the high-gain laser mediumincluding electrical modulation of an electro-optic crystal to performintensity modulation of the laser beam.

[0009] Such a method is an extension of the gain switching method as anoptical resonator's threshold energy state population difference isproportional to the resonator's loss coefficient. In this method, theloss coefficient is modulated to provide intermittent periods when theoptical loss of the high-gain laser medium is prohibitively high tomaintain oscillation. This results is creating an increased energy statethreshold population to sustain oscillation, given the increased loss ofthe high-gain laser medium. Even though the energy state populationwould be sufficiently high for oscillation were the loss coefficient ofthe high-gain laser medium not increased, no optical amplification canoccur during the period when the loss coefficient is elevated.

[0010] When the loss is suddenly decreased during the transition of apulsing cycle of the loss coefficient, the energy state populationbegins to deplete resulting from the decreased optical losses. Thehigh-gain laser medium will amplify the laser light during the periodwhen the energy state population exceeds the threshold condition foroscillation during the period that the loss coefficient is minimum.However, as the population continues to decrease, the population willeventually fall below the newly established energy state threshold foroscillation corresponding to the period of time when the losscoefficient is at its minimum during a modulation cycle.

[0011] These methods of performing electronic switching of either thegain or loss coefficients of the high-gain laser medium often employflashlamp optical pumping. The use of such a light source for performingthe optical pumping presents some undesirable effects whichsignificantly limit performance of the high-gain laser medium inproviding pulses of laser light including the maximum pulse rate of thelaser beam and the intensity with which the optical pumping must beperformed. Such inherent problems may present significant problems forapplications which require high pulse rates and suffer from limitedpower budgets.

[0012] Another problem that is introduced by the utilization offlashlamps to provide optical pumping is the broad spectral width offlashlamp produced light may prove very inefficient in that a largeproportion of the light produced by the flashlamp does not serve togenerate the inverted population of energy states. Flashlamp lightoutside of the spectral density range required for generating theinverted population is simply lost into the high-gain laser medium inthe form of thermal heating. This heating of the high-gain laser mediummay itself produce undesirable effects including beam pointing errorsand self-focusing. The heating of the high-gain laser medium mayincrease to such levels that fracture of the solid-state crystals willlimit the maximum peak or average power.

[0013] The pulse rate at which the laser amplifier may be switched isalso limited by the physical properties of the flashlamps which providethe optical pumping. The electrical switching of the flashlamps is oftenassociated with the thermal heating problems associated with theflashlamps themselves. This upper limit of pulse rate may also bedetermined in part by the intensity level at which the flashlamps mustoperate to generate an inverted energy state population above threshold.For example, if the energy transition of interest is near the peripheryof the spectral density of the flashlamp, the flashlamp may necessitateoperation at a very high power level to generate the invertedpopulation. Such a situation may at the very least limit the duty cycleof the pulse rate to avoid overheating of the flashlamps themselves.

[0014] Additionally, the flashlamps intrinsically possess a start uptime constant before they begin optical pumping. They do not respondinstantaneously with the vertical transition of the electric signalwhich drives them. Consequently, the maximum pulse rate of the opticalamplifier may be limited by the time constant corresponding to the startup of the flashlamps. Another consequence of the intrinsic responselimitations of the flashlamps is a lower limit on the width of the pulsewhich may be generated using such a laser amplification system. Such aproblem stems from the similar characteristic of the flashlamps in thatthey are limited in the speed with which they may switch on and off. Theminimum pulse width which may be generated is often dictated by theminimum time in which the flashlamps may turn on and then turn off,including considering of the start up time constant of the flashlampsand evanescent decay of radiation from the flashlamps when turned off.

[0015] The present invention overcomes or eliminates the problems andlimitations of known systems and methods for detecting high-speedlaser-induced ultrasound to provide a system and method for laser beamamplification from a solid state laser that yields high pulse rates,improved pointing stability, and optionally variable pulse rates fornondestructive laser ultrasonic testing of materials, as well as avariety of other uses.

SUMMARY OF THE INVENTION

[0016] According to one aspect of the present invention, there isprovided a method for generating an amplified laser beam at a high pulserate that includes generating a low-amplification seed laser lightsignal. The method further includes transmitting the low-amplificationseed laser light signal to an amplification component. Thelow-amplification seed laser light signal is amplified in theamplification component by stimulating emissions of the populationinversion that a pumping diode provides. The result of this amplifyingstep is to yield an amplified laser light signal. The amplified laserlight signal is then directed to an output destination. The light signalmay scatter from the output destination and be collected in aninterferometer.

[0017] The present invention provides a system and method for providingamplification of laser light from a solid state laser while maintainingthe physical properties of the laser light by minimizing amplificationinduced distortion. A seed laser possessing desired physical propertiesincluding a single longitudinal mode with a desired linewidth is passedthrough a high-gain laser medium. The high-gain laser medium may takethe form of a diode pumped rod or slab, among others. The high-gainlaser medium is optically pumped using a pumping array of laser diodesdistributed across the high-gain laser medium. The electric currentwhich drives the pumping array may be a time-varying signal whichconsequently provides time-varying optical gain and lasing conditionswithin the laser medium. The amplified laser beam may then be pulsed ata pulse rate corresponding to the frequency of the time-varying signalcomprising the electric current which drives the pumping array of laserdiodes.

[0018] The present invention may be employed in applications whichrequire a particularly narrow or pure spectral density such asapplications involving optical interferometry which often require asingle longitudinal mode having a very stable center frequency andlinewidth. For such applications in which the purity of the initial seedlaser is amplified to generate a pulse stream of laser light having adesired intensity level, duty cycle and pulse rate, an optical isolationassembly may be used to minimize back reflection of the laser light intothe seed laser light source. Undesirable parasitic feedback may corruptthe seed laser source resulting in deleterious performance of the seedlaser including amplitude noise and multimode operation. Such effectsmay be disastrous for those applications requiring a spectrally purelaser source. One method, for providing optical isolation of a laserbeam which minimizes back reflections into the original light sourcecomprises a Faraday rotator and two polarizers which provide for lightpropagation in only one direction through the optical isolation assemblyby using the non-reciprocal rotation of a polarized lightwave providedby a Faraday rotator.

[0019] Additional optical isolation assemblies may be included in theinvention for providing beam splitting of the original laser beam fordirecting the amplified, pulsed output laser beam or directing of theoriginal laser beam through the high-gain laser medium multiple timesfor even greater amplification than a single pass through the high-gainlaser medium.

[0020] The present invention generates an amplified and pulsed laserbeam which possesses similar physical properties of the original seedlaser including a desired center frequency and linewidth for use withinan optical interferometer to perform ultrasonic detection. Additionally,the output intensity of the pulsed, amplified laser beam may bemodulated to extend the dynamic range of detection within an opticalinterferometric system. Electro-optic modulators comprising Pockels orKerr effect crystals provide intensity modulation of a laser beam.

[0021] The present invention may perform optical pumping of a high-gainlaser medium using a pumping array of laser diodes. This method permitsoptical pumping within a very narrow wavelength regime selected by theuse of appropriate laser diodes to minimize optically-induced thermalheating of the laser medium. This advantage is provided primarily by thefact that the appropriate choice of laser diodes which comprise thepumping array may be chosen to optically pump within a specificwavelength regime thereby not incurring significant thermal heating byradiation bombardment of the high-gain laser medium with optical pumpingoutside of the energy transition level of interest.

[0022] The present invention may provide for maximizing the pulse rateand controlling the pulse width of the resulting amplified pulsed laserbeam by performing the optical pumping using a pumping array comprisinga multiplicity of laser diodes. The maximum pulse rate for the presentinvention will be determined largely by the speed at which the laserdiodes may be turned on and turned off and the allowed duty cycle.

[0023] The present invention may minimize amplification induceddistortion of a seed laser beam. Many applications, including opticalinterferometry, require a highly amplified beam with a uniform wavefrontand good pointing stability.

[0024] The present invention may provide for intensity modulation of theoutput laser beam thereby expanding the detection dynamic range withininterferometric systems.

[0025] As such, a system for generating a laser for use with aninterferometer is described. Other aspects, advantages and novelfeatures of the present invention will become apparent from the detaileddescription of the invention when considered in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] For a more complete understanding of the present invention andthe advantages thereof, reference is now made to the followingdescription taken in conjunction with the accompanying drawings in whichlike reference numerals indicate like features and wherein:

[0027]FIG. 1A shows a polarization selective assembly comprising anoptical isolator;

[0028]FIG. 1B shows a polarization selective assembly comprising a fourport optical device providing polarization selective directing of alaser beam;

[0029]FIG. 2 shows a configuration of a laser light source capable ofpreventing optical feedback and varying the intensity variation of thelaser light by rotating a half waveplate;

[0030]FIG. 3 shows another configuration of a laser light source capableof preventing optical feedback and varying the intensity variation ofthe laser light using an electro-optic modulator;

[0031]FIG. 4 shows one possible embodiment of the invention comprising afour pass, dual rod laser media pulsed laser light source;

[0032]FIG. 5 shows another possible embodiment of the inventioncomprising a dual pass, dual rod laser media pulsed laser light source;

[0033]FIG. 6 shows another possible embodiment of the inventioncomprising a four pass, single slab laser medium pulsed laser lightsource;

[0034]FIG. 7 shows another possible embodiment of the inventioncomprising a four pass, dual slab laser media pulsed laser light source;

[0035]FIG. 8 shows another possible embodiment of the inventioncomprising an eight pass, single slab laser medium pulsed laser lightsource;

[0036]FIG. 9A shows one alternative embodiment of the present invention;and

[0037]FIG. 9B shows still a further alternative embodiment of thepresent invention.

[0038]FIG. 10 shows a schematic block diagram of an exemplary embodimentof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0039] Embodiments of the present invention are illustrated in theFIGUREs, like numerals being used to refer to like and correspondingparts of the various drawings.

[0040] The present invention provides a system and method for providingamplification of laser light from a solid state laser while maintainingthe physical properties of the laser light while minimizingamplification induced distortion. A seed laser possessing desiredphysical properties including a single longitudinal mode with a desiredlinewidth is passed through a high-gain laser medium. The centerfrequency of the seed laser source may be chosen appropriately as toperform within specific applications such as optical interferometrywhich require very coherent light. The high-gain laser medium may be,for example, optically pumped using a pumping array of laser diodesdistributed across the high-gain laser medium. The electric currentwhich drives the pumping array may be a time-varying signal whichconsequently provides time-varying optical gain and lasing conditionswithin the laser medium. The amplified laser beam may then be pulsed ata pulse rate corresponding to the frequency of the time-varying signalcomprising the electric current which drives the pumping array of laserdiodes.

[0041]FIG. 1A shows a polarization selective assembly comprising anoptical isolator 10. A typical arrangement of an optical isolator passesa laser beam 18 through a first polarizer 12. The polarized light thenpasses through a Faraday rotator 14. A Faraday rotator provides opticalrotation of a polarized light way in a non-reciprocal fashion. That isto say, polarized light passes through the Faraday rotator will rotatein one and only one direction regardless of the direction of propagationof the laser beam through the material. For example, a laser beam 18traveling through Faraday rotator 14 rotates clockwise as it propagatesin the direction of the arrows of laser beam 18, then a laser beam 18traveling in the opposite direction of the arrows of laser beam 18 willalso rotate clockwise. Faraday rotators 14 are well known to thoseskilled in the art, and may comprise a number of materials includingyttrium-iron-garnet (YIG) or terbium-gallium-garnet (TGG).

[0042] The first polarizer 12 transmits only P-state light from theinput 18. After traveling through Faraday rotator 14, laser beam 18 isrotated 450. Following the Faraday rotator is a half waveplate 16 whichprovides an additional 450 of polarization rotation in the oppositedirection of Faraday rotator 14. Light polarized in a P-state travelingin the direction of the arrows has a net rotation of zero. A halfwaveplate and its operation are well known to those skilled in the art.Light polarized in a p-state traveling in the direction of the arrows oflaser beam 18 through the optical isolator 10 travels unencumbered, yetlight (possibly unpolarized, such as from random scattering) travelingin the opposite direction of the arrows of laser beam 18 through theoptical isolator 10 is blocked. This stems from the fact that theFaraday rotator exhibits non-reciprocal angular rotation of a polarizedlight beam. Light traveling in the opposite direction of the arrows oflaser beam 18 is polarized by the a polarizer 12, then pass through ahalf waveplate 16 and then through the Faraday rotator 14 for a netrotation of 90°. The Faraday rotator 14 will rotate the light one halfof the angular difference between the two polarizers 12 so that whenlight traveling in the opposite direction of the arrows of laser beam 18meets the originally first polarizer 12, at an angle of incidence thatis perpendicular to the polarizer's pass axis thereby completelyblocking back reflections through the optical isolator 10. In effect, anoptical isolator 10 serves as an optical diode or check valve.

[0043]FIG. 1B shows a polarization selective circulator assemblycomprising a four port optical device 11 providing polarizationselective directing of a laser beam. A laser beam 18 enters a polarizingbeam splitter 17 in which polarized light aligned to one axis of thepolarizing beam splitter 17 passes through, and polarized light notaligned to that one axis of the polarizing beam splitter 17 is rejected.For the portion of laser beam 18 which passes through polarizing beamsplitter 17, it then passes through a Faraday rotator 14 followed by ahalf waveplate 16 for directing the laser beam 18. Another polarizingbeam splitter 17 may be used to direct the remaining portion of laserbeam 18 again into two sub-components comprising gain path laser beam 13and aperture path laser beam 15. Four port optical device 11 may be usedto provide steering of components of laser beam 18 in various directionswithin an optical circuit while minimizing back reflections in theopposite direction of the arrows of laser beam 18.

[0044]FIG. 2 shows a first configuration of a laser light source 20capable of preventing optical feedback and varying the intensityvariation of the laser light by rotating a half waveplate. Laser lightsource 22 may comprise a single longitudinal light source operated atcontinuous wave operation. The emitted laser beam 18 from the laserlight source 22 inherently possesses a significant polarization and ahalf waveplate 16 permits the aligning of that polarization along apredetermined axis of polarizer 12 contained within optical isolator 10.Such a configuration is used to minimize any parasitic, undesirable backreflection of laser light into the laser light source 22 which mayresult in deleterious effects such as wavelength drift and linewidthbroadening of the laser light source 22. The half waveplate 16 may beangularly aligned to various angles of incidence of the polarizer 12 tovary the intensity of the laser beam 18 which exits laser light source22 and travels through the optical system. Those skilled in the art willrecognize that a number of optical isolators 10 may be used to decreaseeven further the possibility of back reflected light into the laserlight source 22 by cascading several optical isolators 10.

[0045]FIG. 3 shows another configuration of a laser light source 30capable of preventing optical feedback and varying the intensityvariation of the laser light using an electro-optic modulator. Anoptical isolator 10 may also be used to prevent reflected light fromentering into the laser light source 22 and causing undesired effects asdescribed above. Additionally, an intensity modulator 32 may be placedeither in front of the optical isolator 10 or after for modulating theintensity of the laser beam 18 as it then travels through the remainderof an optical circuit. Those skilled in the art will recognize a widevariety of electro-optic modulators which will serve the function ofintensity modulator 32 including Pockels effect elements utilizing thelinear electro-optic effect and Kerr effect elements utilizing thequadratic electro-optic effect. A very common material candidate for aPockels cells is Lithium niobate (LiNbO₃).

[0046]FIG. 4 shows one possible embodiment of the invention comprising afour pass, dual [could be a single or “n” rods; two is just an example]rod laser media pulsed laser light source 40. Note, however, that source40 may be formed of one or many rods or slabs, as other considerationsmay dictate. This embodiment shows laser light source 20 which emitslaser beam 18 which passes through an isolator 10 and is directed usinga first mirror 42 through a beam expander 46 for broadening the beamwaist of laser beam 18 to minimize the divergence of laser beam 18 as itpropagates through free space given its inherently Gaussian nature.Laser beam 18 then passes through four port optical device 11 in whichlaser beam 18 is directed to the gain path laser beam 13. The gain pathlaser beam 13 passes through two high-gain laser media 48 and thenthrough a phase correction plate and a Faraday rotator 49 where thelinear polarization of laser beam 18 is rotated 45°.

[0047] Typically, high-gain laser media 48 comprising glass materialssuch as Nd³⁺:glass are inherently amorphous and non-birefringent whereassemiconductor materials such as Nd³⁺:YAG might degrade the polarizationstate of laser beam 18 as it passes through them. In the case ofoptically birefringent high-gain laser media, the use of an adjustablewaveplate for phase compensation can improve the performance of thesystem. The phase correction plate compensates for any rodbirefringence, which may be thermally induced. The high-gain laser media48 may comprise any material that will sustain an inverted population ofenergy states when optically pumped. It then reflects off of an endmirror 44 which then passes through the Faraday rotator 49 again whereit is rotated an additional 45°, orthogonal to the polarization of theoriginal laser beam 18.

[0048] The reflected laser beam then passes a second time throughhigh-gain laser media 48 and into four port optical device 11 where itis directed into the direction of aperture path laser beam 15 and thenreflects off a mirror 42 where it passes through an aperture 43 whichhelps to minimize self-oscillations caused by amplified spontaneousemissions from the gain medium. Beam 15 reflects at end mirror 44retracing its path back into four port optical device 11 where it isagain directed to pass through high-gain laser media 48 for a thirdpass. It then travels through to Faraday rotator 49 and to end mirror 44where it is again reflected and retraces its path in passing throughFaraday rotator 49, being converted into the same linear polarization asthe original gain path laser beam 13 in its first pass through high-gainlaser media 48. The gain path laser beam 13 then passes a fourth timethrough high-gain laser media 48 and into the four port optical device11 where it is directed to pass out in the direction of output laserbeam 19.

[0049] Each high-gain optical medium 48 is optically pumped to generatean inverted population of energy states using a pumping array 51 oflaser diodes driven by a diode driver 52 which delivers electric currentto operate the laser diodes of the pumping array 51. To generate pulsesof amplified laser light, a trigger signal 54 is used to drive the diodedriver 52 which operates the pumping array 51 of laser diodes. The pulserate at which the output laser beam 19 may be pulsed is governed mostdirectly by the frequency of the trigger signal 54 which is used topulse the diode driver 52. The switching of the optical pumping resultsin gain switching of the high-gain laser medium 48 which serves toprovide for a pulsing of the conditions in which laser lightamplification may occur. The result is an amplified, pulsed output laserbeam 19.

[0050]FIG. 5 shows another possible embodiment of the inventioncomprising, for example, a dual pass, dual rod laser media pulsed laserlight source 50. This embodiment is strikingly similar to four pass,dual rod laser media pulsed laser light source 40. The main differenceis that there exists no four port optical device 11 is replaced by asingle polarizing beam splitter 17.

[0051]FIG. 6 shows another possible embodiment of the inventioncomprising, for example, a four pass, single slab laser medium pulsedlaser light source 60. This embodiment shows laser light source 20 whichemits laser beam 18 which is isolated from feedback with isolator 10 andis directed through beam expander 46 for broadening the beam waist oflaser beam 18 to minimize the divergence of laser beam 18 as itpropagates through free space given its inherently Gaussian nature.Laser beam 18 then passes through polarizing beam splitter 17 in whichpolarized light aligned to one axis of the polarizing beam splitter 17passes through, and polarized light not aligned to that one axis of thepolarizing beam splitter 17 is redirected in the direction of an outputlaser beam 19. For the portion of laser beam 18 which passes throughpolarizing beam splitter 17, it then passes through a Faraday rotator 14followed by a half waveplate 16 and a second polarizer 17 for directingthe laser beam 18 through high-gain laser media 48. Slab designs passP-state with high efficiency because the face is almost at BrewstersAngle. The high-gain laser media 48 may comprise any material that willsustain an inverted population of energy states when optically pumped.It then reflects off of turning mirror 42. The reflected laser beam thenreflects off of a second turning mirror 42, passes a second time throughhigh-gain laser media 48 and is directed into the direction of laserbeam 15 and then reflects off end mirror 44 before retracing its pathback into the high-gain laser media 48 for a third pass. It thenreflects off mirror 44 to mirror 44 where it is again reflected andretraces its path in passing through high-gain laser media 48 for afourth time and into the polarizer 17 and half waveplate for aligningthe laser beam along a predetermined incidence angle. The laser beam 18then passes a second time through Farady rotator 14. After travelingthrough Faraday rotator 14, the laser beam 18 has been rotated to beorthogonal to the original laser beam. Laser beam 18 enters polarizingbeam splitter 17 where it is directed to pass out in the direction ofoutput laser beam 19.

[0052] In the present example, each high-gain optical medium 48 isoptically pumped to generate an inverted population of energy statesusing pumping arrays 51 of laser diodes driven by a diode driver 52which delivers electric current to operate the laser diodes of the twopumping arrays 51. To generate pulses of amplified laser light, atrigger signal 54 is used to drive the diode driver 52 which operatesthe pumping arrays 51 of laser diodes. The pulse rate at which theoutput laser beam 19 may be pulsed is governed most directly by thefrequency of the trigger signal 54 which is used to pulse the diodedriver 52. The switching of the optical pumping results in gainswitching of the high-gain laser medium 48 which serves to provide for apulsing of the conditions in which laser light amplification may occur.The result is an amplified, pulsed output laser beam 19.

[0053]FIG. 7 shows another possible embodiment of the inventioncomprising a four pass, dual slab laser medium pulsed laser light source70. This embodiment is strikingly similar to four pass, single slablaser medium pulsed laser light source 60, in that it uses the samenumber of diodes as in the FIG. 6 example. The main difference is thatthere exists a second slab laser medium 48 and mirror assemblies 72(either mirrors or coatings) are placed on both of the high-gain lasermedia 48. This may result in a system that is more efficient (per diode)than the system of FIG. 6, but with only a small added cost of thesecond slab.

[0054]FIG. 8 shows another possible embodiment of the inventioncomprising an eight pass, single slab laser medium pulsed laser lightsource 80. This embodiment is strikingly similar to four pass, singleslab laser medium pulsed laser light source 60. The main difference isthat there exists a plurality of turning mirrors 42 to direct the laserbeam through the high-gain laser media 48 eight times and mirrorassembly 72 (either mirrors or coatings) is placed on the high-gainlaser media 48.

[0055]FIG. 9A shows another possible embodiment of the presentinvention. FIG. 9A includes remote seeding of amplifier 54 with a fiberoptic link. Laser light source 20 emits laser beam 18 which is isolatedfrom feedback with optical isolator 10 and directed into input couplingoptics and polarization preserving single-mode fiber optics.Polarization preserving single-mode fiber optic is coupled to amplifier54, which includes output coupling optics. Amplifier 54 (not includingoutput coupling optics) is equivalent to the amplifier section shown inFIG. 6. Amplifier methods illustrated in FIGS. 4, 5, 7 or 8 can be used,as well. Long-term stability of amplifier 54 is improved by de-couplingof laser beam 18. If laser beam 18 laser “walks” but part of the lightstill couples into the polarization preserving single-mode fiber optic,then output of amplifier 54 may only drop a small amount.

[0056]FIG. 9B shows another possible embodiment of the presentinvention. FIG. 9B represents remote seeding of amplifier with internalmodulators. Laser light source 20 emits laser beam 18 which is isolatedfrom feedback with optical isolator 10. Laser beam 18 is input inamplitude modulator, phase modulator and other beam/laser conditioningcomponents. Prior to being input to polarization preserving single-modefiber optics, laser beam 18 is input to input coupling optics. Laserbeam 18 is output from polarization preserving single-mode fiber opticsto output coupling optics. Laser beam 18 is amplified in a mannersimilar to that shown in FIG. 6. Amplifying methods illustrated in FIGS.4, 5, 7 or 8 can be used, as well.

[0057] The present invention provides several benefits includingminimizing thermal heating of the high-gain laser medium by using laserdiodes to perform the optical pumping. Using laser diodes which operatewithin a very narrow wavelength regime minimize optically-inducedthermal heating of the laser medium in that little electromagneticradiation outside of the desired spectrum bombards the high-gain lasermedium as with conventional methods.

[0058] Using laser diodes for optically pumping the high-gain lasermedium provides additional benefits including permitting a faster pulserate, variable pulse rate. By performing optical amplification andpulsing in the manner described above, the present invention alsominimizes amplification induced distortion of a seed laser beam.Consequently, the physical properties of the original seed laser beamare maintained in the resultant amplified, output beam. Manyapplications including optical interferometry require a highlyamplified, spectrally pure output laser beam which the present inventionwill provide. To broaden the dynamic detection range of an opticalinterferometer employing the present invention, the intensity of theoutput laser beam may also be modulated.

[0059]FIG. 10 shows an exemplary embodiment of the amplification systemas used in an optical interferometry application. In this exemplaryapplication, an ultrasound testing system 90 has a sonic energygenerator 92, a laser source 94, and a detection device 96. However, thesystem may be envisaged in various configurations with some, all, ornone of these items. For example, the system 90 may have a laser source94 and a detection device 96.

[0060] The sonic energy generator 92 may take various forms. These formsmay include a laser or a transducer, among others. The sonic energygenerator may generator a sonic energy wave in the object 98. Forexample, a laser may direct a coherent beam of electromagnetic energy atthe object 98. The electromagnetic energy may impart heat energy to theobject causing a thermal expansion. As a result of the expansion, asonic energy wave may by produced in the object 98. However, other meansmay be employed to produce a sonic energy wave such as transducers, orapplied stress, among others.

[0061] The laser source 94 may take the forms as described above.Referring to FIGS. 1B, and 4-8, the beam 19 may be directed at anobject. Returning to FIG. 10, the beam may be directed at an object 98.The beam may be modulated by the sonic energy wave propagating about theobject 98. Further, the beam may reflect from the object to become amodulated reflected beam 102.

[0062] The modulated reflected beam 102 may be collected in a detectiondevice 96. The detection device may, for example, be an interferometer.The interferometer may take many forms. These forms may include aFabry-Perot interferometer, a two-wave mixing interferometer, and a dualdifferential confocal Fabry-Perot interferometer, among others. However,various alternate detection devices may be envisaged including agas-coupled laser acoustic detector and others.

[0063] Although the present invention has been described in detail, itshould be understood that various changes, substitutions and alterationscan be made hereto without departing from the spirit and scope of theinvention as described by the appended claims.

What is claimed is:
 1. A system for measuring sonic energy in a testobject, the system comprising: a source of coherent electromagneticenergy, the source of coherent electromagnetic energy directing at leastone pulse of electromagnetic energy at the test object, the source ofcoherent electromagnetic energy comprising: a low amplification seedlaser light source; and an amplification component, the lowamplification seed laser light source transmitting at least one lowamplification seed laser light signal to the amplification component,the amplification component comprising: at least one amplificationmedium, the low amplification seed laser light signal traversing the atleast one amplification medium; and at least one pumping diode, the atleast one amplification medium amplifying the low amplification laserlight signal by stimulating emissions of a population inversion providedby the at least one pumping diode to generate an amplified laser lightsignal; and an interferometer, the interferometer collecting at leastone scattered light signal produced by a scattering from the object ofthe at least one pulse of electromagnetic energy.
 2. The system of claim1 wherein the low amplification seed laser light signal is amplified byrepeatedly traversing the at least one amplification medium.
 3. Thesystem of claim 1, the system further comprising: a sonic energygenerator, the sonic energy generator generating a sonic energy aboutthe test object.
 4. The system of claim 1, wherein the source ofcoherent electromagnetic energy further comprises: an isolatorassociated with the low amplification seed laser light source, theisolator isolating the low-amplification seed laser light source fromthe amplification component.
 5. The system of claim 1 wherein the atleast one amplification medium is a diode pump rod.
 6. The system ofclaim 1 wherein the at least on e amplification medium is a diode pumpslab.
 7. A method for measuring a sonic energy signal about a testobject, the method comprising: generating a low amplification seed laserlight signal from a low-amplification seed laser light source;transmitting said low-amplification seed laser light signal to anamplification component; amplifying said low-amplification seed laserlight signal in said amplification component by stimulating emissions ofthe population inversion provided by a pumping diode to generate anamplified laser light signal; directing said amplified laser lightsignal to test object; and collecting a scattered light signalassociated with said amplified laser light signal in an interferometer.8. The method of claim 7, wherein said amplifying step further comprisesrepeatedly directing said low-amplification seed laser light signalthrough said amplification medium.
 9. The method of claim 7, whereinsaid directing step further comprises the step of extracting saidlow-amplification seed laser light signal from an isolator associatedwith said amplifier.
 10. The method of claim 7, further comprising thestep of isolating said amplification component from the source of saidlow-amplification seed laser light signal.
 11. The method of claim 7,wherein said amplification component comprises a diode pump rod.
 12. Themethod of claim 7, wherein said amplification component comprises adiode pump slab.
 13. The method of claim 7, wherein said generating stepfurther comprises the step of generating a continuous wavelow-amplification seed laser light signal, and further wherein saidamplifying step further comprises the step of amplifying said continuouswave low-amplification seed laser light signal to generate saidamplified laser light signal having a maximum gain of a predeterminedgain coefficient times the amplification of said continuous wavelow-amplification seed laser light signal.
 14. The method of claim 7,further comprising the step of isolating said low-amplification seedlaser light source from said amplification component for preventingfeedback into said low-amplification seed laser light source.
 15. Themethod of claim 7, further comprising the step of isolating saidamplification component from said output destination for extracting saidamplified laser light signal from said amplification source.
 16. Themethod of claim 7, further comprising the steps of: transmitting saidlow-amplification seed laser light via an optical fiber to an isolatorthat interfaces with said amplification component; and transmitting saidamplified laser light signal through an optical fiber from saidamplification component to said output destination.
 17. The method ofclaim 7, wherein said generating step further comprises the step ofgenerating microsecond pulses of said low-amplification seed laser lightsignal.
 18. The method of claim 7, wherein said generating step furthercomprises the step of generating variable pulse rate pulses of saidlow-amplification seed laser light signal.
 19. A system for measuring asonic energy signal associated with a test object, the systemcomprising: a seed laser light source for providing a laser beam with adesired linewidth; at least one optical isolation assembly placed in thepath of propagation of the laser beam for preventing reflected laserlight feedback into the seed laser light source; a polarizationselective assembly aligned in the path of propagation of the laser beamfor directing a first polarization state of the laser beam in a firstpropagation direction and at least one additional polarization state ofthe laser beam in at least one additional propagation direction; atleast one pumping array comprising a multiplicity of laser diodesdistributed across at least one high-gain laser medium aligned in thepath of propagation of either the first polarization state or the atleast one additional polarization state of the laser beam for opticallypumping the at least one high-gain laser medium for generating apopulation inversion of energy states within the at least one high-gainlaser medium for amplifying the laser beam and generating an outputpulse of laser light; and an interferometer, the interferometercollecting scattered laser light associated with the output pulse oflaser light.
 20. The system of claim 19, further comprising: a diodedriver for delivering an electric current to the multiplicity of laserdiodes for optically pumping the at least one high-gain laser medium;and a trigger signal for pulsing the optical pumping of the high-gainlaser medium by delivering the trigger signal to the diode driver forswitching the electric current delivered to the at least one pumpingarray.